Such systems are used, for example, in combination with heating or cooling plants whose active equipment is to be switched on and off by a parameter sensor, specifically a thermostat, coupled to the control element which for this purpose may be simply a binary switch. In the low-impedance state of this control element, i.e. when the switch is closed, the load is traversed by a large current establishing one operating condition, as by opening a solenoid valve in a conduit through which fuel is fed to an oil burner or the like. In the high-impedance state, i.e. with the switch open, the current is substantially or completely cut off with resulting changeover to the alternate operating condition, namely closure of the solenoid valve in the aforementioned instance. The loads here contemplated include a wide variety of electromagnetic and other devices, among them thermoelements such as bimetallic switches juxtaposed with heating wires traversed by the loop current; lighting fixtures are a further example.
A thermostatic control system for a heating or cooling plant, using only two wires, is described in U.S. Pat. No. 3,948,441. That patent also shows a timer serving for the periodic switchover of the temperature setting of a thermostat to establish different thresholds for daytime and night-time operation, the timer having a motor driven by an accumulator which can be recharged by the current circulating through the loop in the open-circuited state of the thermostat contacts connected thereacross. The thermostats shown in the patent are bimetallic strips in series with adjustable resistors; upon closure of its contacts, such a strip is traversed by virtually the full load circuit.
Even a rechargeable battery or accumulator will not ensure the continued operation of a timer motor when the thermostat contacts have been held closed for an extended period during which no significant charging current could flow. Moreover, the passage of the load current through these contacts tends to subject the thermostat to additional heat which makes the maintenance of a desired mean room temperature more difficult.